In the glass fiber reinforced plastics processing industry, there is always a topic that cannot be avoided, that is, shrinkage and deformation. What causes this to happen?
There are two mechanisms that can cause shrinkage: curing shrinkage and cooling shrinkage.
1. Curing shrinkage
It usually occurs in two stages: during curing (before demolding) and during post-curing (after demolding).
1. Curing shrinkage refers to the volume change of the resin during curing. This curing shrinkage is inevitable, and it is best to occur before demolding from the original mold.
2. After demoulding, a part of curing will also be carried out. These extra shrinkage will cause appearance defects on the mold surface. This is often called "post-curing", but the real deformation is caused by the additional shrinkage during post-curing.
This shows that the selection of suitable low-shrinkage resin is the most effective way to solve the above problems.
The general rule of thumb for curing polyester at room temperature is that when the weight ratio of reinforced glass fiber is 25%, the shrinkage per foot of the straight line is 1/32 inch (1.975px). Compared to fiber-reinforced resins, the gel coat shrinks more, which results in a greater concave curvature of the gel coat surface of the part. For this reason, any large, flat areas should be made slightly convex to avoid depressions in the opposite direction. Small panels often use 1/4 inch (15.875px)/linear foot protrusions.
Second, cooling shrinkage
It is caused by thermal expansion (actually contraction) when the ply is cooled at an unstressed temperature. The unstressed temperature is related to the temperature of the ply when the resin hardens. The greater the gap between the stress-free temperature and the room temperature, the greater the cooling shrinkage.
Therefore, it is possible to reduce the heat release temperature during the curing of the ply as much as possible to control the cooling shrinkage. Minimize post-curing shrinkage through full curing, which may require elevated temperatures. Reducing the heat release temperature and avoiding high temperature as much as possible can also reduce cooling shrinkage. Therefore, the optimal curing scheme depends to a large extent on the heat release temperature of the layer and the surface area weight of the resin.